The present invention relates generally to a superconductive magnetic resonance imaging (MRI) magnet, and more particularly to such a magnet having an open design and cooled by a cryogenic liquid.
MRI systems employing superconductive or other type magnets are used in various fields such as medical diagnostics. Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore and are cooled by liquid helium or a cryocooler. With cryocooler cooling of a closed magnet, the superconductive coil assembly includes a superconductive main coil which is surrounded by a single thermal shield which is surrounded by a vacuum enclosure, and the closed magnet also includes a cryocooler coldhead having a housing attached to the vacuum enclosure, a first stage in solid-conduction thermal contact with the single thermal shield, and a second stage in solid-conduction thermal contact with the superconductive main coil. With liquid-helium cooling of a closed magnet, the superconductive coil assembly includes a superconductive main coil which is at least partially immersed in liquid helium contained in a helium dewar which is surrounded by a dual thermal shield which is surrounded by a vacuum enclosure. It is known to reduce helium boil-off in closed magnets by adding a cryocooler whose first stage is in solid-conduction thermal contact with the outer one of the two spaced-apart thermal shields of the dual thermal shield and whose second stage is in solid-conduction thermal contact with the inner one of the two spaced-apart thermal shields of the dual thermal shield. It is also known to reduce helium boil-off by employing a condenser to re-liquefy the helium vapor, wherein the condenser is in thermal contact with the second stage of a cryocooler coldhead and is in physical contact with the helium vapor in the dewar.
Known open magnets are solid-conduction cryocooler-cooled and typically employ two spaced-apart superconductive coil assemblies with the open space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI imaging. The patient may be positioned in that open space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. A cryocooler coldhead attached to one superconductive coil assembly can cool the second superconductive coil assembly by solid-conduction cooling through the structural posts which attach together the spaced-apart superconductive coil assemblies. The literature is silent on helium-cooled open magnets, presumably because the superconductive main coil structurally cannot be located longitudinally close enough to the open space to allow only a cost-effective amount of superconductor main coil to be used for MRI imaging. What is needed is a helium-cooled open MRI superconductive magnet.